Conventionally, a plurality of types of information discs of different recording formats are handled by one type of disc apparatus for recording information thereto and reproducing information therefrom. In the case of optical discs, for example, a disc apparatus for handling discs such as DVD-ROM discs, DVD-RAM discs, and CD-ROM discs has been put into practice.
This type of disc apparatus is often designed to provide each of elements such as a head, a disc motor, a signal processing LSI and the like with all the functions required for recording and reproduction of various types of discs. Therefore, a condition which is set for one type of disc may disturb a condition for another type of disc, which prevents improvements in performance.
For example, when designing a disc motor, the motor's characteristics are of ten determined in accordance with the rotation conditions of the disc which is to be rotated at the highest rate among all the types of discs used with the disc motor. In this case, in order to reduce the inverse electromotive force of the motor, the number of turns of the coil, for example, is decreased so as to reduce the number of linkage magnetic fluxes between the rotor magnet and the stator coil, thus reducing the generated torque per unit current (torque constant). As a result, inconveniences occur such that, for example, the generation torque of the motor is reduced, and the time required to change the rotation rate of the motor to the target rotation rate is extended.
For example, in the case of a disc apparatus handling discs such as DVD-ROM discs, DVD-RAM discs and CD-ROM discs, the rotation rate required of a spindle motor for recording information to and reproducing information from a DVD-RAM disc is about 3250 rpm at the maximum. In order to increase the speed of a seek operation for the purpose of performing ZCLV (Zoned Constant Linear Velocity) control, a high torque is required for speeds in the range of 1350 rpm (which is a relatively low rate) to 3250 rpm. For recording information to or reproducing information from CD-ROM discs and DVD-ROM discs, there is a strong demand for high speed reproduction. Information on CD-ROMs is often required to be reproduced at a speed of x40 or higher. In order to realize this, a maximum rotation rate of about 9000 rpm is required. Since such a high rotation rate is required only for reproduction, it is not necessary to change the rotation rate during a seek operation owing to a circuit having, for example, a jitter-free structure. Therefore, a very high torque is not needed for a seek operation.
As described above, when the motor coil is designed to provide a high rotation rate and a low torque so as to realize a high reproduction speed which is demanded for CD-ROM discs and DVD-ROM discs, the high torque required for the high speed seek operation for the DVD-RAM discs is not obtained. By contrast, when the motor coil is designed with a larger number of turns to provide a low rotation rate and a high torque for DVD-RAM discs, the rotation rate required for the high speed reproduction for the CD-ROM discs and DVD-ROM discs is not obtained. In this case, these two requirements cannot be provided by one motor.
Japanese Laid-Open Publication No. 10-127086 discloses a technology for making the torque constant of a motor variable by offsetting the translocation timing, for the electric current flowing to the stator coil of the motor, from the regular timing. This technology will be described with reference to FIG. 9.
FIG. 9 shows a structure of a conventional control device 400 for a brushless motor. The control device 400 includes an AC power supply 111, a rectifying and smoothing circuit 112, a switching power supply 113 for outputting a DC voltage VM, a driving control circuit 114 for receiving the DC voltage VM, a pulse width modulation circuit 115, rotor position detectors 108U, 108V and 108W, and a DC motor 123.
The driving control circuit 114 includes a differential amplifier 116 for removing a noise component from three-phase output signals HU′, HV′ and HW′ from the rotor position detectors 108U, 108V and 108W, comparators 117U, 117V and 117W for comparing the three-phase output signals HU′, HV′ and HW′, amplifiers 120U, 120V and 120W for amplifying one of two output signals of the three-phase output signals HU′, HV′ and HW′ which are respectively input to the comparators 117U, 117V and 117W, a logic circuit 118, a current switching circuit 119, a frequency/voltage converter (F/V converter) 121, and amplification ratio varying devices 122U, 122V and 122W.
Here, the amplification ratio of each of the amplifiers 120U, 120V and 120W is K. The comparator 117U compares a multiplication signal HU1 obtained by multiplying the U-phase output signal HU′ from the differential amplifier 116 by amplification ratio K, with the V-phase output signal HV′, and outputs a comparison signal HU2. The comparator 117V compares a multiplication signal HV1 obtained by multiplying the V-phase output signal HV′ from the differential amplifier 116 by amplification ratio K, with the W-phase output signal HW′, and outputs a comparison signal HV2. The comparator 117W compares a multiplication signal HW1 obtained by multiplying the W-phase output signal HW′ from the differential amplifier 116 by amplification ratio K, with the U-phase output signal HU′, and outputs a comparison signal HW2.
The comparison signals HU2, HV2 and HW2 are input to the logical circuit 118. The logical circuit 118 outputs U-, V- and W-phase positive (N-pole) output signals HUU, HVU and HWU to the current switching circuit 119. Simultaneously, the logical circuit 118 outputs U-, V- and W-phase negative (S-pole) output signals HUL, HVL and HWL to the current switching circuit 119.
The current switching circuit 119 turns ON switching power elements TRU1, TRV1, and TRW1 through the U-, V- and W-phase positive poles, and turns ON switching power elements TRU2, TRV2, and TRW2 through the U-, V- and W-phase negative poles. Thus, the current switching circuit 119 sequentially provides a DC voltage VM to the 3-phases 107U, 107V and 107W of the DC motor 123.
The F/V converter 121 converts the output from the differential amplifier 116 into a voltage signal. The amplification ratio varying devices 122U, 122V and 122W vary the amplification ratio K of the amplifiers 120U, 120V and 120W in accordance with the output from the F/V converter 121.
Owing to the above-described structure, when one of the two signals input to each of the comparators 117U, 117V and 117W is respectively amplified by the amplifiers 120U, 120V and 120W at the amplification K where K>1, a lead angle Δθ is given to the three phases 107U, 107V and 107W of the DC motor 123. The lead angle Δθ is set to an optimum value in accordance with the rotation rate of a rotator assembly by the function of the F/V converter 121.
The above-described control device 400 for a brushless motor has the following problems.
When a large lead angle is given to the brushless motor (for example, the DC motor 123), the torque ripple of the brushless motor is increased, and thus an unnecessary vibration of the control device 400 is increased. This makes it difficult to apply the control device 400 to a precision device which is vulnerable to vibrations. The magnitude of vibration is determined by a composite of factors including variance in a vibration adding force occurring due to individual variances among brushless motors and variance in vibration transfer characteristics of the entirety of the control device 400 which is associated with the variance in vibration adding force. In order to restrict the magnitude of the unnecessary vibration to a prescribed value or less with an allowance for all the variance factors, it is necessary that the adjustable range of the lead angle be restricted to be narrow. Thus, the motor's characteristics cannot be significantly changed. When a large lead angle is given to the brushless motor, there occurs another problem that the reliability of the control device 400 for restricting the vibration level to a prescribed level or less is not sufficient.
The level at which the disc apparatus using a brushless motor is allowed to vibrate varies depending on whether an information disc having a high recording density such as a DVD disc is used or an information disc having a low recording density such as a CD-ROM disc is used. As a result, the range of lead angle which can be set for the brushless motor varies. The level at which the disc apparatus using a brushless motor is allowed to vibrate also varies depending on whether a reproduction-only DVD-ROM disc is used or a recordable and reproduceable DVD-RAM disc is used. The range of lead angle which can be set for the brushless motor varies. Such a change in the allowed level of vibration was not conventionally considered.
The above-described disc apparatus complicates the control of variable lead angles, which makes it difficult to simplify the motor control structure. The reason is that the lead angle is set in accordance with the rotation rate. Such a system requires elements including the F/V converter and also requires that the rotation rate be monitored constantly so as to update the lead angle. As a result, a circuit dedicated to setting the lead angle is required, or use of interrupts is often required in order to update the lead angle in the case where a CPU, a DSP or the like is used.
In the case where the target rotation rate is frequently changed and it is necessary to control the actual rotation rate to follow the target rotation rate, the driving current amplitude needs to be controlled while changing the lead angle. Since two parameters are changed simultaneously, the control operation is complicated.
The setting precision of the lead angle is deteriorated due to the variance in the amplitude among the outputs from the three rotor position detectors 108U, 108V and 108W and also due to the waveform distortion of the outputs. When the outputs from the rotor position detectors 108U, 108V and 108W have ideal sine wave signals having equal amplitudes to each other, an accurate lead angle can be set for the following reason. By amplifying one of the two outputs by a constant value and synthesizing the amplified output with the other output, a sine wave signal having a phase which is shifted by a desired amount can be formed. In actuality, however, the outputs from the three rotor position detectors 108U, 108V and 108W have different amplitudes due to, for example, the individual variance among rotor position detectors (for example, hall elements) or changes in characteristics of the rotor position detectors which occur due to temperature. When such signals having different amplitudes are used, the lead angle is different among the U-, V-, and W-phases. This generates an error in the lead angle among the phases. In addition, the outputs from the rotor position detectors 108U, 108V and 108W themselves do not have pure sine waveforms. Due to, for example, the variance in magnetization characteristics of the rotor magnet, the waveforms of the outputs may sometimes be distorted with a high harmonic component superimposed on the output being large. In such a state, the relationship between the amplification ratio K and the lead angle significantly changes from the relationship when a pure sine wave signal is obtained. This deteriorates the setting precision of the lead angle.
It is not necessary that the lead angle can be arbitrary controlled. It is possible to preset the lead angle as one factor with respect to the rotation rate of the motor. This factor generally increases in accordance with the increase in the rotation rate of the motor (∝hall output frequency) in a simple manner. Such a factor is very easily realized by, for example, using a differentiation circuit. Japanese Laid-Open Utility Model Publication No. 62-48198 discloses a technology for setting a lead angle in accordance with the rotation rate of the two-phase pulse motor using the differentiation circuit. Such a technology will be described with reference to FIG. 10.
FIG. 10 shows a structure of a conventional pulse motor driving circuit 500. The pulse motor driving circuit 500 includes a two-phase exciting pulse motor 131, a high resolution encoder 132 connected to the pulse motor 131 for generating an output signal Sf in accordance with a rotation angle θ of the pulse motor 131, an angle calculation circuit 133 for calculating a rotation angle θm of the pulse motor 131 with respect to the magnetic pole based on the output Sf of the encoder 132, a factor generation circuit 134 for generating sine wave signals Ss and Sc having a phase difference of 90 degrees with each other in accordance with the rotation angle θm, multipliers 135 and 136 for varying the amplitudes of the sine wave signals Ss and Sc in accordance with the amplitude of a control signal Si, phase leading circuits 139 and 140, and power amplifiers 137 and 138. Outputs IA and IB of the power amplifiers 137 and 138 are respectively provided to armature coils (not shown) of the pulse motor 131.
FIG. 11 shows an exemplary structure of the phase leading circuits 139 and 140. The phase leading circuits 139 and 140 shown in FIG. 11 each include an amplifier 141, resistors 142 and 143, and a capacitor 144. A phase leading amount of the phase leading circuits 139 and 140 is selected so as to compensate for a phase delay of the magnetic flux with respect to the frequency change in the exciting current.
The position of the rotor (not shown) in the pulse motor 131 is detected by the encoder 132 and the angle calculation circuit 133. The exciting currents IA and IB corresponding to the position are respectively provided to the armature coils.
The amplitudes of the outputs Ss and Sc from the factor generation circuit 134 are represented by expression (1).Ss=sin(θm+π/2)Sc=cos(θm+π/2)  (1).
Where the amplitude of the control signal Si is Io, the magnitudes of the exciting currents IA and IB supplied to the armature coils of the pulse motor 131 are represented by expression (2).IA=Io*sin(θm+π/2)IB=Io*cos(θm+π/2)  (2).
As is clear from expression (2), the exciting currents IA and IB supplied to the respective armature coils have a phase difference of 90 degrees, and the sum thereof (vector synthesis) is constant. Therefore, the rotor generates a constant torque without non-uniformity. The magnitude of the torque is in proportion to the amplitude Io of the control signal Si. Therefore, when the load is zero, the pulse motor 131 is paused if Io is zero. When Io increases, the pulse motor 131 rotates at the rate in accordance with Io.
The electric angle in the synthesis current of the exciting currents IA and IB always has a phase difference of 90 degrees with respect the mechanical angle (rotation angle θ) of the rotator of the pulse motor 131. Therefore, the maximum torque can be generated.
However, when the pulse motor 131 goes into a high rate operation area and thus the frequency of the exciting currents IA and IB increases, the phase of the magnetic flux generated with respect to the driving current is delayed due to the iron loss in the magnetic path or the like. As a result, the maximum torque cannot be generated.
In order to avoid this, the phase leading circuits 139 and 140 are provided in the exciting circuit. When the pulse motor 131 goes into the high rate operation area and thus the frequency of the exciting currents IA and IB increases, the phases of the exciting currents IA and IB are led by the phase leading circuits 139 and 140. Accordingly, the phase delay of the magnetic flux caused by the iron loss in the magnetic path can be compensated for by the leading phase of the exciting currents IA and IB. Thus, it is possible to maintain the phase difference between the mechanical angle and the magnetic flux of the rotor so as to prevent a reduction in the generated torque.
The pulse motor driving circuit 500 has the following problems.
It is assumed that the pulse motor driving circuit 500 is applied to a disc apparatus, and the translocation timing for the electric current flowing to the stator coil of the motor is offset from the regular timing so as to make the torque constant of the motor variable. When it is attempted to realize a high maximum rotation rate required for the high speed reproduction of the CD-ROM discs and the DVD-ROM discs in this state, the phase is excessively led even when the DVD-RAM discs are rotated at a low rate for a high speed seek operation, resulting in a reduction in the torque.
This will be described in detail with reference to FIG. 12. FIG. 12 shows the relationship between the rotation rate and the phase lead angle which is obtained by the phase leading circuit. Characteristic 28 shown in FIG. 12 shows the relationship between the rotation rate and the phase lead angle when the constant of the phase lead circuit is set so as to be appropriate to the high speed reproduction of the CD-ROM discs. Characteristic 28 is obtained when, for example, value R1 of the resistor 143 is set to a relatively small value, the value of the capacitor 144 is set to 0.01 μF, and value R2 of the resistor 142 is set to 21 kΩ in the phase leading circuits 139 and 140.
This setting has main purposes of making the lead angle larger than the lead angle which is necessary to compensate for the current delay caused by the inductance component of the coil, reducing the torque constant of the motor by the effect of the weak field, and restricting generation of an inverse electromotive force so as to improve the maximum rotation rate of the motor. In this manner, the rotation rate of 9000 rpm which is required for x42 reproduction of a CD-ROM disc can be obtained using a disc motor having a no-load rotation rate of 6300 rpm, a starting torque of 190 gcm and a torque constant of 0.17 gcm/mA.
However, characteristic 28 results in a phase lead angle as large as about 10 to 23 degrees with respect to the rotation range of the DVD-RAM discs of 1370 to 3250 rpm. Since the torque constant is reduced by the effect of the weak field, the seek time is undesirably delayed in a DVD-RAM disc which requires the rotation rate to be changed during the seek operation for performing the ZCLV control.
Characteristic 27 shown in FIG. 12 shows the relationship between the rotation rate and the phase lead angle which is appropriate for recording information to or reproducing information from DVD-RAM discs. Characteristic 27 is obtained when R2 is set to a relatively small value, the value of the capacitor 144 is set to 0.01 μF and R1 is set to 6 kΩ. This setting has main purposes of making the lead angle larger so as to be sufficient to compensate for the current delay caused by the inductance component of the coil and maximizing the torque constant of the motor.
Thus, the time of the seek operation of a DVD-RAM disc performed by the same disc motor can be shortened. However, characteristic 27 results in a maximum rotation rate as low as about 5700 rpm, which is not sufficient for the high speed reproduction of information on CD-ROM discs.
As described above, the conditions for the high speed reproduction and the conditions for reduction in the seek time conflict with each other with a single circuit constant setting. It is ideal to realize, for example, characteristic 29 shown in FIG. 12 by which the lead angle rapidly increases when the rotation rate of the motor reaches a prescribed level. However, it is difficult to realize characteristic 29 with the phase leading circuits 139 and 140 shown in FIG. 11.
The present invention for solving the above-described problems has an objective of providing a control device for a brushless motor which is highly reliable against vibrations, is capable of performing easy motor control, and provides a high setting precision of a lead angle; and a disc apparatus using the same.
The present invention also has an objective of providing a disc apparatus for realizing, with one motor and a simple structure, both a high speed seek operation for ZCLV control when recording information to or reproducing information from a DVD-RAM disc and a high speed reproduction operation of a CD-ROM disc and a DVD-ROM disc.